Recently, the necessity of a rewritable recording technique is strongly recognized from the viewpoint of protecting the environment. In accordance with the progress of computer network technology, communication technology, OA machines, recording media, and memory media, paperless technology is being spread at offices and homes. Recording media onto which information can be recorded and erased reversibly utilizing heat, so-called reversible thermal recording media are one of display media used as a substitute for printed materials, and, as a variety of prepaid cards, point cards, credit cards, and IC cards spread, the reversible thermal recording media have been practically used in the applications in which the balance or other recorded information is needed to be visible or readable, and further they are being brought into practical use in the applications of copying machine and printer.
The reversible thermal recording medium and a recording method using the same are described in, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 54-119377, 55-154198, 63-39377, and 63-41186. These are so-called low-molecular substance dispersion type recording media, that is, recording media comprising an organic low-molecular weight substance dispersed in a resin matrix, and the light scattering on the media is changed by thermal history to make the recording layer an opaque or transparent state. Therefore, these media have drawbacks in that the contrast between an image formed portion and an image unformed portion, namely an image portion and the other portion is unsatisfactory, and therefore, only media which are improved in the contrast by providing a reflective layer under the recording layer have been put into practical use.
On the other hand, Unexamined Japanese Patent Application Laid-Open Specification Nos. 2-188293, 2-188294, 5-124360, 7-108761, and 7-188294 disclose a leuco dye type recording medium, that is, a recording medium having a recording layer comprising a leuco dye, which is an electron donating color-forming compound, and a developer dispersed in a resin matrix, and a recording method using the same. In the medium and method, as the developer, an amphoteric compound having an acidic group for developing a leuco dye and a basic group for erasing the colored leuco dye, or a phenolic compound having long-chain alkyl is used. The recording medium and recording method utilize coloring of the leuco dye itself, and therefore, the contrast and recognizability are excellent, as compared to those of the low-molecular substance dispersion type recording medium. As a result, they are being widely used.
In the conventional technique disclosed in the above patent documents, only two colors, specifically, the color of the material for the matrix, i.e., color of the primary surface and the color changed by heat can be displayed. However, in recent years, for improving the recognizability and appearance, there are increasing strong demands of multicolor image display and recording of various data with color identification. For meeting the demands, a number of recording methods have been proposed, in which the above-mentioned conventional technique is applied and a multicolor image is displayed.
Unexamined Japanese Patent Application Laid-Open Specification Nos. 5-62189, 8-80682, and 2000-198275 disclose a recording medium in which layers or particles having different colors are rendered visible or hidden by a low-molecular substance dispersion type recording layer to achieve multicolor display, and a recording method using the same. However, in the recording medium having such a construction, the recording layer cannot completely hide the colors of the underlying layers and the color of the matrix is seen through, so that a high contrast cannot be obtained.
In Unexamined Japanese Patent Application Laid-Open Specification Nos. 8-58245 and 2000-25338, there are disclosures concerning reversible thermal multicolor recording media using a leuco dye, but these reversible thermal multicolor recording media have repeating units having different colors in the surface, and therefore the area ratio of the individual hues to the actually recorded portion is small. Thus, the recorded image is very dark or has a low contrast.
In Unexamined Japanese Patent Application Laid-Open Specification Nos. 6-305247, 6-328844, 6-79970, 8-164669, 8-300825, 9-52445, 11-138997, 2001-162941, and 2002-59654, there are disclosures concerning reversible thermal multicolor recording media having a construction in which recording layers using leuco dyes having different coloring temperatures, decoloring temperatures, and cooling rates are formed so that they are separated and independent from one another.
However, these reversible thermal multicolor recording media have problems in that temperature control is difficult using a recording heat source, such as a thermal head, and an excellent contrast cannot be obtained, so that an occurrence of fogging cannot be avoided. Further, it is very difficult to control the recording of multicolor, i.e., three colors or more merely by changing the heating temperature using a thermal head or the like and/or the cooling rate after heating.
On the other hand, in Unexamined Japanese Patent Application Laid-Open Specification No. 2001-1645, there is a disclosure concerning a recording method using a reversible thermal multicolor recording medium having a construction in which recording layers using leuco dyes are formed so that they are separated and independent from one another, in which only an arbitrary recording layer is heated and colored by light-to-heat transformation using a laser beam. In this method, only a desired recording layer can be colored by the effect of the wavelength selectivity of the light-to-heat transforming layer, possibly solving the problem of fogging accompanying the conventional reversible multicolor recording media.
However, the light-to-heat transforming layer and the recording layer are individually formed, and therefore the number of the constituent layers is large, causing the production process to be complicated. Further, the method has a problem in that energy generated by the light-to-heat transformation in the laser radiation is not efficiently transferred to the recording layer, so that satisfactory coloring cannot be achieved, thus prolonging the time for recording.
As mentioned above, there are strong demands on the multicolor thermal recording and studies are vigorously conducted, but a practically satisfactory recording medium or recording method has not yet been found.